1. Field of the Invention
This invention relates generally to the field of phantoms used in ultrasound evaluation to test the accuracy of and to calibrate ultrasonic equipment, and more particularly, to a method which can be used to quantitatively determine the imaging effectiveness of ultrasound devices at various depths and to a device which closely mimics the transmission pattern of ultrasonic waves propagating through one or more portions of the human body.
2. Description of the Prior Art
Although ultrasound equipment has been in use for a number of years, very few devices have been proposed which can be used to quantitatively assess the integrated performance level of the equipment, thus making it difficult to calibrate and check the equipment on a regular basis. This is especially true of diagnostic ultrasonic equipment used in the medical field. This type of diagnostic equipment should be calibrated using phantoms which are designed to mimic sound wave propagation characteristics of the human body. Ideally, such phantoms should be capable of approximating soft tissue with respect to (a) speed of sound, (b) attenuation coefficient, and (c) scattering coefficients. Additionally, such phantoms should also mimic other conditions which may be present at the investigation sight, such as blood flow or the systaltic movement of blood vessels.
A number of attempts have been made in the past to provide effective diagnostic ultrasound phantoms that mimic soft tissue. In a paper presented at the Second International Symposium on Ultrasonic Tissue Characterization in Gaithersburg, Md (1977), Eggleton describes work with soft plastics in the form of plastisols. At the same symposium, P.L. Carson, L. Shaboason and D.E. Dick presented a paper on work with polyurethane polymers. Also at the symposium, P. Edwards presented a paper describing work with gelatins.
More recently, U.S. Pat. No. 4,277,367, issued to Madsen et al., teaches a phantom which contains water-based pharmaceutical gelatins containing uniform distributions of various substances and known concentrations of alcohol. The substances act as scatterers within the gelatine matrix and may include solid particles of graphite, talc, pumice or polyethylene microspheres. Alternatively, the substances may include liquid particles of vegetable oil or kerosine. However, one drawback to this type of ultrasound phantom stems from the distribution of the scattering substance. Specifically, the scattering substances have a tendency to settle out of suspension, thus altering the ultrasound properties of the phantom. For example, when the gelatine is at temperatures over 90.degree. F., suspended graphite begins to settle. Furthermore, it is often difficult to achieve and maintain a uniform dispersion of scattering substance throughout the gelatin. Also, many gelatins employed can be unstable under certain conditions, primarily due to bacterial attacks on the gel and ambient temperature variations, thus leading to degradation of the gelatin which can result in unpredictable operation.
Finally, it is often difficult to incorporate zones within the gelatin to mimic cysts or the like. Typically, a gelatine phantom is prepared by inserting a plug into uncongealed gelatin containing scattering material. Once the gelatin has hardened into a matrix, the plug is removed and gelatin containing no scattering material is allowed to congeal within the void. When scanned with an ultrasound system, waves should ideally pass unreflected through the plug in the same way they would pass through a cyst in the human body. One difficulty with this method, however, is the formation of an interface between the gel and the plug, resulting in an artifact. Specifically, a thin skin typically forms on the surface of the gelatin at the interface between the plug and the hardened gelatin matrix. During operation, this skin layer will cause a reflection resulting in the appearance of a ring down artifact within the cyst.
To overcome many of these problems, the use of open-cell reticulated foam material as the scatter matrix has been proposed. U.S. Pat. No. 4,286,455 issued to Ophir et al. teaches a phantom which includes a substantially air-tight enclosure having a quantity of reticulated synthetic resin foam material and a salt water solution therein. The ultrasonic characteristics of the phantom can be varied by using different foam materials and/or liquids. Further, localized zones having different ultrasonic qualities for mimicking various normal and pathological tissues can be provided by hollowed, cut-out regions, such as cylindrical cavities, within the foam material. Inserts with various scattering characteristics are provided within the hollowed regions. Ultrasonic waves directed through the phantom will be back-scattered by the fibers within the sponge while the forward wave will pass unscattered through the cut-out regions. If a plug has been inserted into the cut-out region, the echo of back-scattered waves from the plug will differ from that of the surrounding matrix.
However, even the reticulated foam type of phantom can only simulate imaging for static conditions within the human body. For phantoms which are intended to simulate dynamic conditions within the human body, such as blood flow or systaltic movement of vessels, other devices have been suggested. One method of simulating blood flow is through the use of a string phantom which generally utilizes a string riding between two pulleys in a water bath. As the pulleys rotate, the motion of the string is intended to appear as flowing blood when imaged with an ultrasound device. A disadvantage to this type of phantom is that scatter resulting from a string is not a realistic condition because the string acts as a "perfect" point target in a water bath. Thus the echo which is analyzed by the system is too ideal and does not indicate how the system will operate under more realistic conditions.
Another type of flow phantom utilizes fluid flow through tubes to simulate blood flow. A scatter material is mixed with fluid and then back-scatters sonic waves in much the same way as blood platelets reflect ultrasonic waves in blood. However, it is often difficult to maintain scatter particles in a uniform suspension to yield a constant scatter level. This is especially true at low flow rates because scatter particles tend to settle out of suspension. As scatter particles settle and become less uniformly distributed, it becomes difficult to mimic accurate flow rates over a wide range of flow velocities. Although pumping and stirring devices have been utilized to maintain uniform conditions, the devices are often expensive, greatly add to the complexity of the phantom and are only marginally effective.
Each of the above described phantoms can be used to approximate ultrasonic wave propagation through living tissue, allowing evaluation of ultrasound devices. This evaluation is based, in part, on the ultrasound system's ability to focus an ultrasonic beam. The ability to focus an ultrasonic beam, in part characterizes the imaging ability of the ultrasound system because those devices with better focusing ability produce higher quality images. The information provided by the phantom, however, goes beyond the evaluation of the beam forming characteristics of the system alone. That is, since the measurement is ultimately based upon the image formed by the total ultrasound system, the resulting quantitative measurement reveals the imaging characteristics of the total system. In this way, the measurement technique depicts the integrated response of the ultrasound system.
In an ideally focused ultrasound beam, energy is emitted and/or received in only the direction in which the beam is directed. That is, in the transmit mode, energy would only propagate out along an infinitely narrow line directed away from the transducer. Conversely, in the receive mode, the ultrasound system would only be sensitive to energy entering the transducer aperture from the direction in which the transducer is aimed. In other words, the transducer would be totally insensitive to any energy entering the aperture at an angle other than that at which the energy was emitted from the transducer. In effect, an ideal beam profile would appear to be a spacial impulse function directed in the steered beam direction.
In a non-ideally focused ultrasound beam, the beam profile will tend to converge and then diverge, as the beam propagates, resulting in a beam profile that has some finite width and is characterized by a central main lobe and weaker side lobes. The integral of the main lobe, which is represented by a first discreet amount of energy, and the integral of the side lobe energy, represented by a second discrete amount of energy, are both simultaneously detected by the transducer as a summation. To the extent that the main lobe of the ultrasound beam pattern is sufficiently narrow, then the main lobe can be considered as approximating a spacial impulse function. In this case, the signal energy contained in the beam side lobes can be considered as an unwanted signal or noise. An image formed under these conditions is the summation of the desired main lobe energy and the undesired side lobe "noise" energy. In the event that the main beam is broad, then energy falling beyond a specified spacial angle would also be considered noise even though it was technically part of the main lobe. The less focused the ultrasound beam, the greater the proportional amount of energy found outside the desired beam direction, and hence, the lower the signal to noise ratio.
It is readily recognized in the field of ultrasonic imaging that the width of the main lobe and the height of the side lobes of a beam formed by ultrasonic waves play a significant role in determining clinical image quality. It is known in the prior art that the energy level found in the side lobes of ultrasound beams increases with beam degradation. Generally, commercially available cyst phantoms are used to qualitatively evaluate how well an ultrasound beam can detect a cyst or sonolucent region within a phantom, i.e., how well the device can focus an ultrasonic beam at a given depth. Since no echo should be generated in cystic regions because of the absence of scatter material, any ultrasonic energy that does appear to be reflected from cystic regions is actually caused by side lobe insonification of the adjacent regions.
In other words, although ideally a transducer which propagates an ultrasonic beam through a cyst should not receive any signal because there is no scatter material in the region of the cyst, in reality the transducer will receive side lobe energy (or energy from the periphery of the main lobe) from regions adjacent to the straight path of the beam. The effect is that there does appear to be some back-scatter emanating from the region of the cyst. For example, on the display screen a true cyst at a predetermined depth should appear black because there should be no sonic reflection from the cyst. Because the ultrasound device is not capable of perfectly focusing an ultrasonic beam, the image of the cyst on the display screen will appear to have some level of echo, i.e., the cyst would have a salt and pepper appearance on the screen.
Thus, although an image may be generated by the ultrasonic scan of a phantom, it is often difficult to determine which portion of the image can be attributed to the desired signal from the steered direction and which part is due to off angle energy contained in either the side lobes or the edges of a broad main lobe. With only qualitative information about a device's imaging capabilities, it becomes difficult to conduct diagnostic system evaluations. Additionally, the lack of quantitative data makes it difficult to compare system performance.
Quantitative methods of ultrasound evaluation have been proposed utilizing a phantom with isolated regions of different scatter intensity within the matrix. In these procedures, an isolated region within a phantom is scanned with a first ultrasound device and the resulting image is compared to the image produced by a second ultrasound device. It has been suggested that the difference in the images is a quantitative means which can be interpreted to determine the imaging capabilities of the ultrasound devices in question. This method, however, produces only relative results because there is no absolute standard to which the information can be compared. Additionally, this method is subject to variables which are functions of the conditions surrounding each individual test, such that there are no standardized test conditions. Therefore, any information gleaned from the resulting ultrasound images may potentially be even further skewed by dissimilar test conditions. For example, the relative contrast difference between scatter regions is a function of the ultrasound system's gain, the signal center frequency, the signal band width, the spectral shape and the depth of the region within the phantom. Thus the results are only partially a function of the beam pattern and are also comprised of parameters which are not constant with each test.
Therefore, it is desirable to provide a method for quantifying ultrasonic image data such that the data is based only on the beam pattern and not on the conditions particular to each test. The quantified information should be in a form that is standardized to allow comparison with other ultrasound systems. Additionally, it would be desirable to provide a phantom which can simulate blood flow at various velocities and through different diameter vessels. Lastly, it would be desirable to provide a phantom which can simulate systaltic conditions within a living body.